1. Field of the Invention
The present invention relates to a cylindrical lithium ion secondary battery having a functional center pin, and more particularly, to a cylindrical lithium secondary battery capable of reducing the current interruption time in case of overcharging.
2. Description of Related Art
Typically, a cylindrical lithium ion secondary battery includes a cylindrical electrode assembly having a center pin coupled thereto, a cylindrical can to which the electrode assembly is coupled, an electrolyte injected into the can to enable lithium ions to move, and a cap assembly coupled to the can to prevent the electrolyte from leaking and the electrode assembly from escaping.
Cylindrical lithium ion secondary batteries typically have a capacity of 2000-2400 mA and are usually used for laptop computers, digital cameras, camcorders, etc., which require a large capacity of electric power. For example, a number of cylindrical lithium ion secondary batteries may be connected in series and in parallel as desired and are assembled in a hard pack of a predetermined shape with a protective circuit mounted thereon to be coupled to an electronic appliance and used as the power supply.
A cylindrical lithium ion secondary battery may be manufactured as follows: a negative electrode plate coated with a negative electrode active material, a separator, and a positive electrode plate coated with a positive electrode active material are laminated together. An end of the resulting laminate is coupled to a rod-shaped winding shaft and is wound into an approximately cylindrical shape to form an electrode assembly. Then, the electrode assembly is inserted into a cylindrical can and a center pin is inserted into the electrode assembly. An electrolyte is injected into the cylindrical can and a cap assembly is coupled to the cylindrical can to complete an approximately cylindrical lithium ion secondary battery.
In order to prevent the cylindrical lithium ion secondary battery from exploding in case of overcharging, the battery is provided with a safety vent which deforms when internal pressure rises due to overcharging, and a circuit board which interrupts the current as the safety vent deforms. The safety vent and the circuit board are also referred to together as current interruption devices (CIDs) and are included as part of the cap assembly.
The operation of the safety vent and the circuit board of a cylindrical lithium ion secondary battery will now be described in more detail.
When a cylindrical lithium ion secondary battery is overcharged, the electrolyte evaporates from the electrode assembly, causing increased resistance. In addition, lithium precipitates and deformation begins to occur in the central region of the electrode assembly. The increase of resistance in the electrode assembly may cause an abrupt rise in battery temperature.
When a battery is overcharged, cyclo hexyl benzene (CHB) and biphenyl (BP) (electrolyte additive) may decompose and generate gas, rapidly increasing the internal pressure of the battery. Such internal pressure deforms the safety vent toward the exterior of the battery. As a result, the circuit board positioned on the safety vent is fractured and interrupts the current. Specifically, the wiring pattern formed on the circuit board is fractured and current no longer flows. Such interruption of current prevents the battery from exploding or catching fire.
When the internal pressure of a battery rises above a critical level due to overcharging, the safety vent itself is torn off and internal gas is allowed to escape to the exterior.
A void volume or dead volume generally exists inside the battery. In particular, the space between the electrode assembly and the cap assembly or the space inside the center pin may be referred to as a void volume. The existence of a void volume is thought to delay the current interruption time and degrade the stability of the battery.
It is known in the art that, when the safety vent inside the battery deforms (or the circuit board fractures) at a pressure of about 5-11 kgf/cm2 and the void volume is about 2 ml, for example, the amount of gas necessary for deformation of the safety vent is about 10-22 ml, although there may be some variance depending on the type of battery. However, even when CHB completely decomposes, which is included in the electrolyte at a ratio of 0.7% based on calculation, about 4.116 ml of gas is generated and, even when 0.3% of BP completely decomposes, about 1.833 ml of gas is generated. In addition, about 1.5 ml of gas is additionally generated in the degassing process. The total sum of gas from three different sources, however, is no more than about 7.449 ml and applies a force of about 3.75 kgf/cm2 to the safety vent. In summary, although a pressure of about 5-11 kgf/cm2 is necessary to deform the safety vent or break the circuit board in the case of overcharging, the void volume can actually provide a pressure of about 3.75 kgf/cm2 at most. As a result, the safety vent is not fractured or the fracture time is delayed, thereby delaying the current interruption time. The longer current interruption is delayed, the more likely the battery will explode or catch fire due to overcharging. Although the amount of gas generated during overcharging may increase by increasing the amount of CHB or BP, which are electrolyte additives, there is a trade-off between degradation of capacity and quality of the battery.